PhraseX is a program that extracts noun phrase strings from text. It does so by referring to the syntactic structure provided by the SPECIALIST minimal commitment parser, which relies on the SPECIALIST Lexicon as well as the Xerox stochastic tagger (Cutting et al. 1992). The output produced is in the tradition of partial parsing (Hindle 1983, McDonald 1992, Weischedel et al. 1993) and concentrates on the simple noun phrase--what Weischedel et al. (1993) call the "core noun phrase," that is, a noun phrase with no modification to the right of the head. Several approaches provide similar output based on statistics (Church 1988, Zhai 1997, for example), a finite-state machine (Ait-Mokhtar and Chanod 1997), or a hybrid approach combining statistics and linguistic rules (Voutilainen and Padro 1997).

The SPECIALIST parser is based on the notion of barrier words (Tersmette et al. 1988), which indicate boundaries between phrases. After lexical look-up and resolution of category label ambiguity by the tagger, complementizers, conjunctions, modals, prepositions, and verbs are marked as boundaries. Subsequently, boundaries are considered to open a new phrase (and close the preceding phrase). Any phrase containing a noun is considered to be a (simple) noun phrase, and in such a phrase, the right-most noun is labeled as the head; all other items (other than determiners) are labeled as modifiers. An example of the output from the SPECIALIST parser is given in (2) for the input in (1).

(1) Kupffer cells from halothane-exposed guinea pigs carry 
    trifluoroacetylated protein adducts.

(2)

[
   [
     mod([lexmatch(['Kupffer']),inputmatch(['Kupffer']),tag(noun)]),
     head([lexmatch([cells]),inputmatch([cells]),tag(noun)]) ],
   [
     prep([lexmatch([from]),inputmatch([from]),tag(prep)]),
     mod([lexmatch([halothane]),inputmatch([halothane]),tag(noun)]),
     punc([inputmatch([-])]),
     mod([lexmatch([exposed]),inputmatch([exposed]),tag(adj)]),
     head([lexmatch(['guinea pigs']),inputmatch([guinea,pigs]),
                                                      tag(noun)]) ],
   [
     verb([lexmatch([carry]),inputmatch([carry]),tag(verb)]) ],
   [
     mod([lexmatch([trifluoroacetylated]),
                       inputmatch([trifluoroacetylated]),tag(adj)]),
     mod([lexmatch([protein]),inputmatch([protein]),tag(noun)]),
     head([lexmatch([adducts]),inputmatch([adducts]),tag(noun)]),
     punc([inputmatch(['.'])]) ]
]

The underspecified structure produced by the SPECIALIST parser serves as the basis for the extraction of noun phrase strings by PhraseX. In addition to the simple noun phrase (labeled as "simp" in output), PhraseX identifies two additional structures. One of these is the complex noun phrase in which a head is followed by contiguous prepositional phrases to its right ("macro"); The first preposition in this structure can be anything, but all the rest must be "of". The second structure is not a canonical syntactic phenomenon, but may be important for information processing. Such a phrase includes all the content words that occur in a sentence either to the left or the right of a finite verb ("mega"). Examples of these strings as extracted from the syntactic structure in (2) are given in (3).

(3) 00000000|simp|kupffer cells 
    00000000|simp|halothane exposed guinea pigs 
    00000000|simp|trifluoroacetylated protein adducts 
    00000000|macro|kupffer cells from halothane exposed guinea pigs 
    00000000|mega|kupffer cells from halothane exposed guinea pigs 
    00000000|mega|trifluoroacetylated protein adducts 

References

Ait-Mokhtar, Salah and Jean-Pierre Chanod. (1997). Incremental finite-state parsing.
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Church, Kenneth W. (1988). A stochastic parts program and noun phrase parser for unrestricted
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Cutting Douglas R., J. Kupiec, Jan O. Pedersen, and P. Sibun. (1992). A practical part-of-speech
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Hindle, Donald. (1983). Deterministic parsing of syntactic non-fluencies. Proceedings of the 21st
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McDonald, David D. (1992). Robust partial parsing through incremental, multi-algorithm processing.
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Tersmette KWF, Scott AF, Moore GW, Matheson NW, and Miller RE. (1988). Barrier word
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Vourtilainen, Atro and Lluis Padro. (1997). Developing a hybrid NP parser. Proceedings of the
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Weischedel, Ralph, Marie Meteer, Richard Schwartz, Lance Ramshaw, and Jeff Palmucci.
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Zhai, Chengxiang. (1997). Fast statistical parsing of noun phrases for document indexing. Proceedings
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